PLC for distributed control and distributed control system

ABSTRACT

A PLC for distributed control includes a storage unit that stores common-data specifying information for specifying common data shared by a corresponding PLC and another PLC, a receiving unit that receives the common-data specifying information from another PLC, and a collating unit that collates the common-data specifying information stored in the storage unit with the common-data specifying information received by the receiving unit.

BACKGROUND OF THE INVENTION

The present invention relates to a PLC (programmable controller) fordistributed control and a distributed control system including aplurality of PLCs for distributed control connected to one another.

Recently, in order to control mechanical equipments positioned at aplurality of places for factory automation, a distributed control systemincluding a plurality of PLCs controlling mechanical equipmentsconnected to one another has been known (for example, see PatentDocument 1).

Such a distributed control system according to the related art uses amethod in which adjacent PLCs are connected to each other by a pluralityof I/O wiring lines and a plurality of signals are transmitted throughthe I/O wiring lines. Since safety PLCs for securing the safety ofcontrol objects are required to correctly transmit a plurality ofcontrol signals, such as interlock signals, according to a plurality ofcontrol objects to one another, the above-mentioned method is veryeffective.

-   Patent Document 1: JP-A-2002-358106

However, in the above-mentioned method, an operator is required tomanually check the connection between the PLC terminals connected to theindividual I/O wiring lines in order to determine whether communicationby each wiring line is correctly performed, resulting in a lowmaintenance property. Further, since the plurality of I/O wiring linesshould be provided between the PLCs, the number of PLCs increases, andthus the cost increases.

SUMMARY OF THE INVENTION

An object of the invention is to provide a PLC for distributed controlthat improves the maintenance property while securing the safety of thecommunication with another PLC constituting a distributed controlsystem.

Another object of the invention is to provide a distributed controlsystem that improves the maintenance property while securing the safetyof the communication between PLCs for distributed control.

According to a first aspect of the invention, a PLC for distributedcontrol that constitutes a distributed control system together withanother PLC includes: a storage unit that stores common-data specifyinginformation for specifying common data shared by a corresponding PLC andanother PLC; a receiving unit that receives the common-data specifyinginformation from another PLC; and a collating unit that collates thecommon-data specifying information stored in the storage unit with thecommon-data specifying information received by the receiving unit. Inthe PLC for distributed control, it is possible to automatically, notmanually, determine whether the common data is correctly transmittedbetween the corresponding PLC for distributed control and another PLC.Therefore, it is possible to secure the safety of communication betweenthe PLC for distributed control and another PLC and to improve amaintenance property.

In the PLC for distributed control according to a first aspect, thecollating unit may perform the collation of the common-data specifyinginformation when the distributed control system starts or restarts.Therefore, it is possible to secure the safety of the communicationbetween the PLC for distributed control and another PLC whenever thedistributed control system starts or restarts.

Further, in the PLC for distributed control according to the firstaspect, the common-data specifying information may include informationon an address where the common data is assigned in a frame used forcommunication of the distributed control system. For this reason, eventhough a plurality of common data are assigned to a plurality ofaddresses of one frame and are simultaneously transmitted thereto, it ispossible to secure the safety of the transmission on the basis of theabove-mentioned theory. Therefore, it is possible to improve themaintenance property and to reduce the cost by reducing the number ofsignal lines while securing the safety of the communication between thePLC for distributed control and another PLC.

Furthermore, the PLC for distributed control according to the firstaspect may constitute the distributed control system together with aplurality of PLCs. In this case, the common-data specifying informationmay include identification information for identifying the plurality ofPLCs. With this construction, it is possible to determine which of theplurality of PLCs the common-data specifying information is receivedfrom. Therefore, the PLC for distributed control can accurately performthe collation for the common-data specifying information received fromother PLCs by using the collating unit.

According to a second aspect of the invention, a distributed controlsystem includes a plurality of PLCs for distributed control according tothe first aspect connected to one another. In the distributed controlsystem, the plurality of PLCs for distributed control are mutuallycooperated with one another. Therefore, it is possible to secure thesafety of the communication among the PLCs for distributed control andto remarkably improve the maintenance property of the overalldistributed control system.

In the distributed control system according to the second aspect, eachof the PLCs for distributed control may be a safety PLC for ensuring thesafety of a control object connected to the corresponding PLC fordistributed control. Since the safety of the communication among thePLCs for distributed control is secured as described above, it ispossible to correctly transmit the common data related to the safety ofthe control objects among the PLCs for distributed control. Therefore,it is possible to secure the safety of all the control objects connectedto each of the PLCs for distributed control with high reliability.

Further, in the distributed control system according to the secondaspect, the common-data specifying information may be input to each ofthe PLCs for distributed control together with a program to be executedby the corresponding PLC for distributed control. With thisconstruction, the sequence program to be executed by each of the PLCsfor distributed control and the common-data specifying information canbe freely changed in response to the requirement of the systemspecification, thereby expanding the versatility of the distributedcontrol system.

Furthermore, in the distributed control system according to the secondaspect, the common-data specifying information may be associated witheach other and may be output on a ladder diagram representing thecontent of programs to be executed by the PLCs for distributed control.With this construction, an operator of the distributed control systemcan check the common-data specifying information among the PLCs fordistributed control together with the ladder diagram with eyes, therebyfurther improving the maintenance property.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating an initial operation according to afirst embodiment of the invention.

FIG. 2 is a block diagram showing a distributed control system accordingto the first embodiment and a second embodiment.

FIGS. 3A to 3C are schematic diagrams illustrating the operation of thedistributed control systems according to the first and secondembodiments.

FIG. 4A 1 to 4C are schematic diagrams illustrating the operation of thedistributed control system according to the second embodiment.

FIG. 5 is a flow chart illustrating an initial operation according tothe second embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, a plurality of embodiments of the invention will bedescribed with reference to the accompanying drawings.

First Embodiment

FIG. 2 shows a distributed control system according to a firstembodiment of the invention. A distributed control system 1 is a networksystem including a plurality of safety PLCs connected in a ringtopology. The distributed control system 1 performs distributed controlon a plurality of output devices 4 on the basis of input informationfrom a plurality of input devices 2 so as to secure the safety of theoutput devices 4.

In the distributed control system 1, the maximum number of safety PLCscapable of being connected to the distributed control system 1 is set to24. In this embodiment, three safety PLCs 10, 11, and 12 are connectedto the distributed control system 1. Each of a plurality of signal lines14 connecting the safety PLCs 10, 11, and 12 is an optical fiber line ora conductive line. Communication among the safety PLCs 10, 11, and 12through the signal lines 14 uses frames each of which is composed of abit string with a predetermined bit length. Here, a bit address isassigned as address information to bit data constituting one frame bythe operation of a communication module 30 to be described below. Inthis embodiment, frame data is received and transmitted between two ofthe safety PLCs 10, 11, and 12. In other words, the distributed controlsystem 1 according to the first embodiment is a broadcast networksystem.

Each of the safety PLCs 10, 11, and 12 constituting the distributedcontrol system 1 has a CPU module 20, a communication module 30, and anI/O module 40.

The CPU module 20 includes a microcomputer as the main constituent andis connected to the communication module 30 through a bus 21. A memory22 of the CPU module 20 stores a sequence program written in a ladderdiagram language. The CPU module 20 controls reception and transmissionof control instructions or data through the bus 21 by allowing a CPU 24to execute the sequence program. Further, in this embodiment, the CPUmodule 20 of the safety PLC 10 instructs the communication module 30 ofthe safety PLC 10 to output a control instruction for controllingcommunication between the safety PLCs 11 and 12, thereby controlling thecommunication of the whole system.

The communication module 30 includes a microcomputer as the mainconstituent. The communication module 30 is connected to the CPU module20 through the bus 21, and is also connected to the I/O module 40through a bus 31. The communication module 30 has an interface 32connected to the signal lines 14 between the communication module 30 andthe other PLCs. A memory 34 of the communication module 30 stores acommunication program. The communication module 30 executes thecommunication program by a CPU 36 so as to control reception andtransmission of data or control instructions through the buses 21 and 22and the interface 32 according to a control instruction received fromthe CPU module 20.

The I/O module 40 is connected to an emergency button, a predeterminednumber of input devices 2 such as safety sensors, a motor, and apredetermined number of output devices 4 such as robots. The I/O module40 transmits data indicating the state of the input devices 2 to thecommunication module 30. Further, the I/O module 40 performs, forexample, power supply control on the output devices 4 according to thecontrol instruction received from the communication module 30.

In the distributed control system 1 according to the first embodiment,I/O devices 50, 51, and 52 are connected to interfaces 26 provided inthe CPU modules 20 of the safety PLCs 10, 11, and 12, respectively. Eachof the I/O devices 50, 51, and 52 has a computer 53, an input device 54,a monitor 55, and a printer 56.

An operator inputs, to a computer, exchange numbers of safety PLCsconnected to the corresponding computer 53 by operating the input device54 such as a keyboard or a mouse. Here, the exchange numbers areidentification information for identifying the safety PLCs 10, 11, and12, and exchange numbers ‘00’, ‘01’, and ‘02’ are set to the safety PLCs10, 11, and 12 in this embodiment, respectively.

Further, the operator inputs, to the computer 53, a sequence program tobe executed by the CPU module 20 of the safety PLC connected to thecorresponding computer 53. Here, the sequence program is a programaccording to a ladder diagram describing a data reception/transmissionsequence by the PLC connected to the computer 53, for example, a programaccording to one of FIGS. 3A to 3C corresponding to the exchange numberof the PLC connected to the computer 53.

In the ladder diagrams shown in FIGS. 3A to 3C, a parameter ‘d-a’associated with a coil represents an exchange number ‘d’ (one of 00 to02) of the destination PLC of the transmission data and a bit address‘a’ (one of L00 to L7F) of the transmission data in the frame. Further,in the ladder diagrams shown in FIGS. 3A to 3C, a parameter ‘D-A’associated with a contact point represents an exchange number ‘D’ (oneof 00 to 02) of the destination PLC of the reception data and a bitaddress ‘A’ (one of L00 to L7F) of the reception data in the frame.Therefore, the parameters ‘d-a’ and ‘D-A’ can be considered ascommon-data specifying information for specifying data to be shared bythe safety PLCs 10, 11, and 12. In this embodiment, the operator inputsthe common-data specifying information together with the sequenceprogram to the computer 53 by operating the input device 54.

The exchange number, the sequence program, and the common-dataspecifying information (the parameters ‘d-a’ and ‘D-A’) input to thecomputer 53 as described above are stored in a memory 57 of the computer53. Then, the exchange number, the sequence program, and the common-dataspecifying information are input from the computer 53 to the CPU module20 of the safety PLC connected to the corresponding computer in responseto the operator's instruction and are then stored in the memory 22. Thecommon-data specifying information stored in the memory 22 of the CPUmodule 20 is transferred to the memory 34 of the communication module 30through the bus 21 in an initial operation to be described below.Further, in the initial operation, the parameter ‘d-a’ of thetransferred information related to the transmission data to betransmitted to another safety PLC is output by the communication module30 and is transmitted to a safety PLC with the exchange numbercorresponding to the value ‘d’ of the parameter ‘d-a’.

An input operation by the computer 53 and the input device 54 isperformed as described above. Now, an output operation of the computer53 to the monitor 55 and the printer 56 will be described. First, inresponse to an operator's instruction, the computer 53 reads thesequence program and the common-data specifying information stored inthe memory 57. Then, the computer 53 associates the common-dataspecifying information with each other on a ladder diagram representingthe content of the sequence program, as shown in FIGS. 3A to 3C, andthen outputs the common-data specifying information from at least one ofthe monitor 55 and the printer 56. Here, output from the monitor 55means displaying, on the monitor 55, the ladder diagram in which thecommon-data specifying information are associated with each other.Further, output from the printer 56 means printing the ladder diagram inwhich the common-data specifying information is associated with eachother on a medium such as a sheet.

Next, an initial operation that is performed by the distributed controlsystem 1 immediately after starting or immediately after restarting willbe described. In this embodiment, the distributed control system 1starts by turning on a main power supply of the corresponding system.Furthermore, the distributed control system 1 restarts by resetting themain power supply of the corresponding system.

As shown in FIG. 1, when the initial operation starts, each of thesafety PLCs, 10, 11, and 12 transfers the common-data specifyinginformation from the memory 22 of the CPU module 20 to the memory 34 ofthe communication module 30 (Step S1). Subsequently, the communicationmodule 30 of the safety PLC 10 reads the common-data specifyinginformation related to data shared by the safety PLCs 11 and 12 from thememory 34, and transmits the read common-data specifying information tothe safety PLCs 11 and 12 (Step S2). The common-data specifyinginformation transmitted from the safety PLC 10 to the safety PLC 11 is aparameter ‘01-a’ related to data transmitted from the safety PLC 10 tothe safety PLC 11, for example, a parameter ‘01-L00’ shown in FIG. 3A.Further, the common-data specifying information transmitted from thesafety PLC 10 to the safety PLC 12 is a parameter ‘02-a’ related to datatransmitted from the safety PLC 10 to the safety PLC 12, for example, aparameter ‘02-L04’ shown in FIG. 3A.

After receiving the common-data specifying information from the safetyPLC 10, the communication module 30 of each of the safety PLCs 11 and 12collates the received common-data specifying information with thecommon-data specifying information stored in the memory 34 of thecorresponding communication module (Step S3). More specifically, thecommunication module 30 of the safety PLC 11 checks whether the ‘a’value of the parameter ‘01-a’ of the reception information from thesafety PLC 10 corresponds to the ‘A’ value of the parameter ‘00-A’related to data received from the safety PLC 10 of the informationstored in the memory 34. For example, when the parameter ‘01-L00’ shownin FIG. 3A is received and the parameter ‘00-L00’ shown in FIG. 3B isstored in the memory 34 of the safety PLC 11, the communication module30 of the safety PLC 11 determines that the parameters correspond toeach other. In the safety PLC 12, the communication module 30 checkswhether the ‘a’ value of the parameter ‘02-a’ of the receptioninformation from the safety PLC 10 corresponds to the ‘A’ value of theparameter ‘00-A’ related to data received from the safety PLC 10 of theinformation stored in the memory 34. For example, when the parameter‘02-L04’ shown in FIG. 3A is received and the parameter ‘00-L04’ shownin FIG. 3B is stored in the memory 34 of the safety PLC 11, thecommunication module 30 of the safety PLC 12 determines that theparameters correspond to each other.

When determining that the reception information and the informationstored in the memory 34 correspond to each other in Step S3, thecommunication module 30 of each of the safety PLCs 11 and 12 determinesthat a normal state in which data is correctly transmitted between thesafety PLC 10 and the corresponding safety PLC has been confirmed (StepS4). On the other hand, when determining that the reception informationand the information stored in the memory 34 does not correspond to eachother, the communication module 30 of each of the safety PLCs 11 and 12determines that an abnormal state in which data cannot be correctlytransmitted between the safety PLC 10 and the corresponding safety PLChas been confirmed (Step S4).

After both the safety PLCs 11 and 12 confirm the normal state in StepS4, the communication module 30 of the safety PLC 11 reads thecommon-data specifying information related to data shared by the safetyPLCs 11 and 12 from the memory 34, and transmits the read common-dataspecifying information to the safety PLCs 10 and 12 (Step S5). Here, thecommon-data specifying information transmitted from the safety PLC 11 tothe safety PLC 10 is a parameter ‘00-a’ related to data to betransmitted from the safety PLC 11 to the safety PLC 10, for example,parameters ‘00-L42’ and ‘00-L43’ shown in FIG. 3B. Also, the common-dataspecifying information transmitted from the safety PLC 11 to the safetyPLC 12 is a parameter ‘02-a’ related to data to be transmitted from thesafety PLC 11 to the safety PLC 12.

When having received the common-data specifying information from thesafety PLC 11, the communication module 30 of each of the safety PLCs 10and 12 collates the received information with the common-data specifyinginformation stored in the memory 34 of the corresponding module (StepS6). More specifically, the communication module 30 of the safety PLC 10checks whether the ‘a’ value of the parameter ‘00-a’ of the informationreceived from the safety PLC 11 corresponds to the ‘A’ value of theparameter ‘01-A’ related to data received from the safety PLC 11 of theinformation stored in the memory 34. For example, when the parameter‘00-L42’ shown in FIG. 3B is received and the parameter ‘01-L42’ shownin FIG. 3A is stored in the memory 34 of the safety PLC 10, or when theparameter ‘00-L43’ shown in FIG. 3B is received and the parameter‘01-L43’ shown in FIG. 3A is stored in the memory 34 of the safety PLC10, the communication module 30 of the safety PLC 10 determines that theparameters correspond to each other. In the safety PLC 12, thecommunication module 30 checks whether the ‘a’ value of the parameter‘02-a’ of the information received from the safety PLC 11 corresponds tothe ‘A’ value of the parameter ‘01-A’ related to data received from thesafety PLC 11 of the information stored in the memory 34.

When determining that the received information and the informationstored in the memory 34 correspond to each other in Step S6, thecommunication module 30 of each of the safety PLCs 10 and 12 determinesthat the normal state in which data is correctly transmitted between thesafety PLC 11 and the corresponding safety PLC has been confirmed (StepS7). On the other hand, when determining that the received informationand the information stored in the memory 34 does not correspond to eachother, the communication module 30 of each of the safety PLCs 10 and 12determines that the abnormal state in which data cannot be correctlytransmitted between the safety PLC 11 and the corresponding safety PLChas been confirmed (Step S7).

After both the safety PLCs 10 and 12 confirm the normal state in StepS7, the communication module 30 of the safety PLC 12 reads thecommon-data specifying information related to data shared by the safetyPLCs 10 and 11 from the memory 34, and transmits the read common-dataspecifying information to the safety PLCs 10 and 11 (Step S8). Here, thecommon-data specifying information transmitted from the safety PLC 12 tothe safety PLC 10 is the parameter ‘00-a’ related to data to betransmitted from the safety PLC 12 to the safety PLC 10, for example,parameters ‘00-L44’ and ‘00-L45’ shown in FIG. 3C. Also, the common-dataspecifying information transmitted from the safety PLC 12 to the safetyPLC 11 is the parameter ‘01-a’ related to data to be transmitted fromthe safety PLC 12 to the safety PLC 11.

The communication module 30 of each of the safety PLCs 10 and 11 havingreceived the common-data specifying information from the safety PLC 12collates the received information with the common-data specifyinginformation stored in the memory 34 of the corresponding communicationmodule (Step S9). More specifically, the communication module 30 of thesafety PLC 10 checks whether the ‘a’ value of the parameter ‘00-a’ ofthe information received from the safety PLC 12 corresponds to the ‘A’value of the parameter ‘02-A’ related to data received from the safetyPLC 12 of the information stored in the memory 34. For example, when theparameter ‘00-L44’ shown in FIG. 3C is received and the parameter‘02-L44’ shown in FIG. 3A is stored in the memory 34 of the safety PLC10, or when the parameter ‘00-L45’ shown in FIG. 3B is received and theparameter ‘02-L45’ shown in FIG. 3A is stored in the memory 34 of thesafety PLC 10, the communication module 30 of the safety PLC 10determines that the parameters correspond to each other. In the safetyPLC 11, the communication module 30 checks whether the ‘a’ value of theparameter ‘01-a’ of the information received from the safety PLC 12corresponds to the ‘A’ value of the parameter ‘02-A’ related to datareceived from the safety PLC of the information stored in the memory 34.

When determining that the received information and the informationstored in the memory 34 correspond to each other in Step S9, thecommunication module 30 of each of the safety PLCs 10 and 11 determinesthat the normal state in which data is correctly transmitted between thesafety PLC 12 and the corresponding safety PLC has been confirmed (StepS10). On the other hand, when determining that the received informationand the information stored in the memory 34 does not correspond to eachother, the communication module 30 of each of the safety PLCs 10 and 11determines that the abnormal state in which data cannot be correctlytransmitted between the safety PLC 12 and the corresponding safety PLChas been confirmed (Step S10).

After both the safety PLCs 10 and 11 confirm the normal state in StepS10, that is, when all of the safety PLCs 10, 11, and 12 confirm thenormal state, the communication module 30 of the safety PLC 10 instructsthe CPU module 20 of the safety PLC 10 to start normal communication(Step S11). Meanwhile, when any one of the safety PLCs 10, 11, and 12confirms the abnormal state, the communication module 30 of the safetyPLC 10 instructs the CPU module 20 of the safety PLC 10 to prohibit thenormal communication (Step S12).

According to the above-mentioned first embodiment, when the systemstarts or restarts, each of the safety PLC 10, 11, and 12 determineswhether data is normally transmitted between the corresponding safetyPLC and the other safety PLCs on the basis of the collation of its ownthe common-data specifying information with the common-data specifyinginformation received from other PLCs. In other words, in the firstembodiment, it is possible to automatically determine whethercommunication among the safety PLCs 10, 11, and 12 is correctlyperformed without the operator. Therefore, it is possible to ensure thesafety of the communication among the safety PLCs 10, 11, and 12 and toimprove the maintenance property of the overall system. In particular,since the safety of the communication among safety PLCs 10, 11, and 12is secured, it is possible to ensure the safety of the output devices 4connected to the safety PLCs 10, 11, and 12 with high reliability.

Further, according to the first embodiment, the parameters ‘d-a’ and‘D-A’, which are the common-data specifying information, include the bitaddresses ‘a’ and ‘A’ to which common data is assigned, in the frameused for the communication of the distributed control system. For thisreason, even though a plurality of common data are assigned to aplurality of addresses of one frame and are simultaneously transmittedthereto, it is possible to secure the safety of the transmission on thebasis of the above-mentioned theory. Therefore, it is possible toimprove the maintenance property and to reduce the cost by reducing thenumber of signal lines while securing the safety of the communicationamong the safety PLCs 10, 11, and 12.

Furthermore, according to the first embodiment, the parameters ‘d-a’ and‘D-A’, which are the common-data specifying information, include theexchange numbers ‘d’ and ‘D’ for identifying each of the safety PLCs 10,11, and 12. The exchange numbers make it possible for the safety PLCs10, 11, and 12 to determine which of the safety PLCs 10, 11, and 12 thereceived common-data specifying information is received from. Therefore,the safety PLCs 10, 11, and 12 can accurately perform theabove-mentioned collation for common-data specifying informationreceived from other PLCs.

In addition, according to the first embodiment, the sequence program tobe executed by the safety PLCs 10, 11, and 112 and the common-dataspecifying information can be input to the safety PLCs 10, 11, and 12through the I/O devices 50, 51, and 52. Therefore, the sequence programand the common-data specifying information can be freely changed inresponse to the requirement of the system specification, therebyexpanding the versatility of the distributed control system 1.

Furthermore, according to the first embodiment, the I/O devices 50, 51,and 52 can output the common-data specifying information on the ladderdiagram representing the content of the sequence program to be executedby the safety PLCs 10, 11, and 12, with the common-data specifyinginformation associated with each other. Therefore, the operator cancheck from the output results of the I/O devices 50, 51, and 52 witheyes whether the common-data specifying information correctly specifiesthe common data to be transmitted among the safety PLCs 10, 11, and 12,as shown in the ladder diagram. As a result, it is expected to furtherimprove the maintenance property.

The above-mentioned safety PLCs 10, 11, and 12 correspond to ‘a PLC fordistributed control’ and ‘other PLCs’ described in the appended claims.Further, the memories 22 and 34 of each of the safety PLCs 10, 11, and12 correspond to ‘storage units’ described in the appended claims. Thecommunication module 30 of each of the safety PLCs 10, 11, and 12correspond to ‘a receiving unit’, ‘a collating unit’, and a‘transmitting unit’ described in the appended claims.

Second Embodiment

A second embodiment of the invention is a modification of the firstembodiment. In the second embodiment, the same parts as those in thefirst embodiment are denoted by the same numerals, and thus adescription thereof will be omitted.

A distributed control system 1 according to the second embodiment is amaster-slave network system that has a safety PLC 10 serving as a masterand safety PLCs 11 and 12 serving as slaves. Therefore, in the secondembodiment, frame data is received and transmitted between the safetyPLCs 10 and 11 and between the safety PLCs 10 and 12, while frame datais not received and transmitted between the safety PLCs 11 and 12.

In I/O devices 50, 51, and 52 of the second embodiment, common-dataspecifying information (parameters ‘d-a’ and ‘D-A’) input to a computer53 is stored in a memory 57 of the computer 53, is processed into aframe format by the computer 53, and is stored in the memory 57.

More specifically, the computer 53 connected to the safety PLC 10generates a frame in which data corresponding to bit addressesrepresented by the values ‘a’ and ‘A’ of the parameters ‘01-a’ and‘01-A’ related to common data shared by the safety PLCs 10 and 11 arerepresented by ‘use (U)’ and data corresponding to the other bitaddresses are represented by ‘non-use (N)’. Therefore, the computer 53connected to the safety PLC 10 generates a frame shown in FIG. 4A 1 inwhich bit data having bit addresses ‘L00’, ‘L42’, and ‘L43’corresponding to parameters ‘01-L00’, ‘01-L42’, and ‘01-L43’ shown inFIG. 3A are ‘U’. The computer 53 also generates a frame in which datacorresponding to bit addresses represented by the values ‘a’ and ‘A’ ofparameters ‘02-a’ and ‘02-A’ related to the common data shared by thesafety PLCs 10 and 12 are ‘U’, and data corresponding to the otheraddresses are ‘N’. Therefore, the computer 53 connected to the safetyPLC 10 generates a frame shown in FIG. 4A 2 in which bit data having,for example, bit addresses ‘L04’, ‘L44’, and ‘L45’ corresponding toparameters ‘02-L04’, ‘02-L44’, and ‘02-L45’ shown in FIG. 3A are ‘U’.

The computer 53 connected to the safety PLC 11 generates a frame inwhich data corresponding to bit addresses represented by the values ‘A’and ‘a’ of the parameters ‘00-A’ and ‘00-a’ related to common datashared by the safety PLCs 10 and 11 are represented by ‘use (U)’ anddata corresponding to the other bit addresses are represented by‘non-use (N)’. Therefore, the computer 53 connected to the safety PLC 11generates a frame shown in FIG. 4B in which bit data having bitaddresses ‘L00’, ‘L42’, and ‘L43’ corresponding to parameters ‘00-L00’,‘00-L42’, and ‘00-L43’ shown in FIG. 3B are ‘U’.

The computer 53 connected to the safety PLC 12 generates a frame inwhich data corresponding to bit addresses represented by values ‘A’ and‘a’ of parameters ‘00-A’ and ‘00-a’ related to the common data shared bythe safety PLCs 10 and 12 are ‘use (U)’ and data corresponding to theother addresses are ‘non-use (N)’. Therefore, the computer 53 connectedto the safety PLC 12 generates a frame shown in FIG. 4C in which bitdata having, for example, bit addresses ‘L04’, ‘L44’, and ‘L45’corresponding to parameters ‘00-L04’, ‘00-L44’, and ‘00-L45’ shown inFIG. 3C are ‘U’.

The common-data specifying information processed into the frame formatas described above is input from the computer 53 to the CPU module 20 ofthe safety PLC connected to the computer 53 together with the exchangenumbers and the sequence program, and is stored in the memory 22 of thecorresponding safety PLC. In an initial operation to be described below,the common-data specifying information stored in the memory 22 in theframe format is transferred to the memory 34 of the communication module30 through the bus 21 and is then transmitted from the communicationmodule 30 to another safety PLC.

The initial operation according to the second embodiment is performed asshown in FIG. 5. That is, when the initial operation starts, each of thesafety PLCs 10, 11, and 12 transfers the common-data specifyinginformation of the frame format from the memory 22 of the CPU module 20to the memory 34 of the communication module 30 (Step S101).

Subsequently, the communication module 30 of the safety PLC 10 reads,from the memory 34, a part of the common-data specifying information ofthe frame format that relates to the common data shared by the safetyPLCs 10 and 11 and is shown in, for example, FIG. 4A 1, and transmitsthe read information to the safety PLC 11 (Step 102). Then, thecommunication module 30 of the safety PLC 10 reads, from the memory 34,a part of the common-data specifying information of the frame formatthat relates to the common data shared by the safety PLCs 10 and 12 andis shown in, for example, FIG. 4A 2, and transmits the read informationto the safety PLC 12 (Step S102).

The communication module 30 of each of the safety PLCs 11 and 12 havingreceived the common-data specifying information of the frame format fromthe safety PLC 10 collates the received common-data specifyinginformation of the frame format with the common-data specifyinginformation of the frame format stored in the memory 34 (Step S103).More specifically, the communication module 30 of the safety PLC 11collates bit data of the frame received from the safety PLC 10 as thecommon-data specifying information with bit data of the frame stored inthe memory 34 as the common-data specifying information so as todetermine whether the bit data coincide with each other for each sameaddress. For example, when a frame in which addresses where bit data are‘U’ are ‘L00’, ‘L42’, and ‘L43’ as shown in FIG. 4A 1 is received and aframe in which addresses where bit data are ‘U’ are ‘L00’, ‘L42’, and‘L43’ as shown in FIG. 4B is stored, the communication module 30 of thesafety PLC 11 determines that the bit data coincide with each other.Further, the communication module 30 of the safety PLC 12 collates bitdata of the frame received from the safety PLC 10 as the common-dataspecifying information with bit data of the frame stored in the memory34 as the common-data specifying information so as to determine whetherthe bit data coincide with each other for each same address. Forexample, when a frame in which addresses where bit data are ‘U’ are‘L04’, ‘L44’, and ‘L45’ as shown in FIG. 4A 2 is received and a frame inwhich addresses where bit data are ‘U’ are ‘L04’, ‘L44’, and ‘L45’ asshown in FIG. 4C is stored, the communication module 30 of the safetyPLC 12 determines that the bit data coincide with each other.

On the basis of the collation results in Step S103, the communicationmodules 30 of the safety PLCs 11 and 12 perform determination, similarto Step S4 of the first embodiment (Step S104). When both thecommunication modules 30 of the safety PLCs 11 and 12 confirm the normalstate in Step 104, the communication module 30 of the safety PLC 11reads, from the memory 34, a part of the common-data specifyinginformation of the frame format that relates to the common data sharedby the safety PLCs 10 and 11 and is shown in, for example, FIG. 4B, andtransmits the read information to the safety PLC 10 (Step S105).

The communication module 30 of the safety PLC 10 having received theframe format common-data specifying information from the safety PLC 11collates the received frame format common-data specifying informationwith the frame format common-data specifying information stored in thememory 34 according to Step S103 (Step 106). On the basis of thecollation result, the communication module 30 of the safety PLC 10performs determination, similar to Step S7 of the first embodiment (StepS107).

When the communication module 30 of the safety PLC 10 confirms thenormal state in Step S107, the communication module 30 of the safety PLC12 reads, from the memory 34, a part of the frame format common-dataspecifying information that relates to the common data shared by thesafety PLCs 10 and 12 and is shown in, for example, FIG. 4C, andtransmits the read information to the safety PLC 10 (Step S108).

The communication module 30 of the safety PLC 10 having received thecommon-data specifying information of the frame format from the safetyPLC 12 collates the received common-data specifying information of theframe format with the common-data specifying information of the frameformat stored in the memory 34 according to Step S103 (Step S109). Onthe basis of the collation result, the communication module 30 of thesafety PLC 10 performs determination based on Step S10 in the firstembodiment (Step S110).

When the communication module 30 of the safety PLC 10 confirms thenormal state in Step S110, it instructs the CPU module 20 of the safetyPLC 10 to start normal communication, similar to Step S11 of the firstembodiment (Step S111). Meanwhile, when any one of the safety PLCs 10,11, and 12 confirms the abnormal state in Steps S104, S107, and S110, itinstructs the CPU module 20 thereof to prohibit the normal communicationbased on Step S12 in the first embodiment (Step S112).

As described above, each of the safety PLCs 10, 11, and 12 according tothe second embodiment can also determine the normal state or theabnormal state between the corresponding safety PLC and another safetyPLC by collating the common-data specifying information stored in thecorresponding safety PLC with the common-data specifying informationreceived from another safety PLC. Therefore, the second embodiment canobtain the same effects as the first embodiment.

The embodiments of the invention have been described above, but theinvention is not limited thereto.

For example, in the first embodiment, each of the safety PLCs 10, 11,and 12 may transmit the parameter ‘D-A’ related to the received data ofthe corresponding safety PLC to another safety PLC and may collate theparameter ‘D-A’ with the parameter ‘d-a’ stored in the correspondingsafety PLC in relation to the transmitted data.

In the first and second embodiments, the invention is applied to thedistributed control system 1 connected to a plurality of PLCs eachsecuring the safety of a control object. However, the invention can beapplied to a distributed control system connected to a plurality of PLCseach having a function other than the function of securing the safety ofa control object. Further, a topology of a distributed control system towhich the invention is applied may be a bus or star other than the ringdescribed in the first and second embodiments, regardless of the type ofPLC.

1. A PLC for distributed control that constitutes a distributed controlsystem together with another PLC, the PLC comprising: a storage unitthat stores common-data specifying information for specifying commondata shared with another PLC; a receiving unit that receives thecommon-data specifying information from another PLC; and a collatingunit that collates the common-data specifying information stored in thestorage unit with the common-data specifying information received by thereceiving unit.
 2. The PLC according to claim 1, wherein the collatingunit performs the collation of the common-data specifying informationwhen the distributed control system starts or restarts.
 3. The PLCaccording to claim 1, wherein the common-data specifying informationincludes information on an address where the common-data is assigned ina frame used for communication of the distributed control system.
 4. ThePLC according to claim 1, wherein the distributed control system iscomposed of a plurality of PLCs, and the common-data specifyinginformation includes identification information for identifying theplurality of PLCs.
 5. A distributed control system comprising: aplurality of PLCs for distributed control according to any one of claims1 to 4 that are connected to one another, wherein the plurality of PLCsare mutually cooperated with one another.
 6. The distributed controlsystem according to claim 5, wherein each of the PLCs includes atransmitting unit that transmits the common-data specifying informationto another PLC.
 7. The distributed control system according to claim 5,wherein each of the PLCs is a safety PLC for ensuring safety of acontrol object connected to the corresponding PLC.
 8. The distributedcontrol system according to claim 5, wherein the common-data specifyinginformation is input to each of the PLCs together with a program to beexecuted by the corresponding PLC.
 9. The distributed control systemaccording to claim 5, wherein the common-data specifying informationitems are associated with each other on a ladder diagram representingthe content of programs to be executed by the PLCs.